The invention relates to a power supply control system for delivering power concurrently to multiple furnaces, and more particularly to a control system for delivering power in a controlled, predetermined apportioned manner to two furnaces simultaneously from a single power supply and a single reactive capacitor station.
Power supplies for selectively or alternatively heating multiple induction furnaces are known. One system for powering two melting furnaces alternately that has been often used in the past is referred to as a "butterfly operation." In such an operation, a single power supply supplies energy alternately to two furnaces operating as a holding furnace and a melting furnace. The first furnace holds molten metal and requires only enough power to control the metal temperature so that it remains molten. The second furnace holds metal to be melted as rapidly as possible. The power supply is normally located in such a position that its output can be readily switched from one furnace to the other. Initially the power supply is connected to the melting furnace and delivers as much power to the load as possible. The temperature of the metal in the holding furnace is monitored. When the molten metal temperature in the holding furnace reaches a minimum, the power to the melting furnace is shut off, the output of the power supply is connected to the holding furnace and the holding furnace is energized. The power is kept on for the holding furnace until the metal temperature reaches a maximum limit. At that time the power to the holding furnace is shut off, the output of the power supply is connected to the melting furnace and the melting furnace is energized. This operation is repeated throughout the melting cycle whenever the temperature control of the holding furnace demands power.
The result of the butterfly operation is poor temperature control in the holding furnace and poor utilization of power to the melting furnace. In addition, the power supply must turn off/on at each switching to allow transfer of output connections, which means that during transfer neither furnace receives power.
In terms of efficiency of use of power, an improved system is shown in U.S. Pat. No. 5,272,719 which discloses a power supply system for simultaneously melting metal and holding molten metal for casting operations with a single power supply. The power supply is connected to the furnaces through a switching network wherein a plural output power supply comprises at least one rectifier section having an output and a plurality of high frequency inverter sections equal to the number of separate induction furnaces.
A particular problem with this system is that each furnace requires its own high-frequency inverter system which necessarily includes expensive tank and filter capacitors and the associated switch circuitry for controlling delivery of power to each of the furnaces. In addition, power consumption to activate respective capacitor tank circuits for each of the furnaces is increased over a system avoiding a need for multiple tank circuits.
The present invention contemplates a new and improved multi-furnace control system which overcomes the above-referred to problems and others to provide a furnace control system for simultaneously powering multiple furnaces such as a holding furnace and a melting furnace from the same capacitor station at preselected individual furnace power levels, during which all operation of the power supply and furnace capacitors is accomplished within safe limits.